Itachi beats almost most.
1. He pulled Sasuke to the end, while being very very sick (He died due to it, so he was already at his limit). You try fighting like that, when you are about to die. A situation like that leaves you at 20% power. A dieing person ain't the same as a fit one. He also killed Orochimaru in about 5 seconds. Not to mention his Sasonukee is impenetrable, and penetrates anything.
2. Itachi vs Jirayai. This wouldn't be a fight, as Itachi still had an endgoal of protecting Konoha. Why would he fight someone who has the same goals as him? Even if he would fight him, I wouldn't see Itachi go all out ot kill Jirayai, but rather just to keep himself alive and avoid the fight.
3. Granny Tsunade is by far stronger then we think. She is for a REASON one of the selected to become hokage. I am 100% sure we haven't seen 100% of her potential strength yet. I hope we will in future
I believe Itachi was one of the most powerfull characters in the manga. We most likely have never seen Itachi even fight at full power (atleast not all the potential power he had when he died). Then again, he was one of the good characters, so they don't have the urge to fight en destroy, which often hids their true powers. One facts I believe that show Itachi that he never really pushed out his true power, is for one that he was never suprised, he always had a reaction if anyone pulled out something dangerous on him. We never seen a fight with Itachi fighting with honest intention of killing .... How can one pull out his true power then ?
4. Madara is by far not the strongest person. He maybe used to be, but as Itachi once said :"He is only a shell of his forme self". It speaks for itself. He still seems good at manipulating people for his goals, but I don't think he has the power capable to fight some of the top-people currently.
5. Orochimaru was extremely powerfull ... but I believe he wasn't a match for Itachi. Orochimaru pulled off a kill on the Thirth, but was close
My list of strongest people would be :
1. Fourth (Hokage at his age speaks for itself)
2. First
3. Itachi
4. Orochimaru (Yes Oro above Sasuke, even though Sasu beat him. The level of strenght of Oro was a lot higher still as you could see in the advanced jutsu. Oro was dumb enough to try catch Sasuke in a genjutsu, which is the specialty of the Sharingan ... a very dumb mistake)
5.Thirth
6. Pain (None knows how strong he really is)
7. Jiraiya
8. Second
9. Sasuke
10. Madara
But then again, it all depends who matches who in a fight.
We don't know anything about the new Sasuke atm , but I think he might be skyrocketed in the top 5.